Add features for ml-kem variants

[franziskuskiefer]

This PR

• adds ml-kem variants features (mlkem768 etc) to allow enabling only a single variant
• adds separate extraction for mlkem768 in a header only mode with cg.yaml
• makes c extraction in c.sh more flexible to use all these things

The Rust code changes are

• use a simple for loop in rejection sampling
• drop strings from unimplemented macros in sha3 to avoid extraction them to C
• disable the multiplexing module when extracting C

[franziskuskiefer]

Shall we document the various features and use-cases in libcrux-ml-kem/README.md?

Features can be nicely documented in the rust docs. That's where people also expect them and where they don't get out of sync. We need to do a pass on docs before publishing anyway.